How dry is CNG and why you may need a dryer for your CNG Fueling station?

All natural gas contains some amount of water. This is most evident when you first light your stove top burner and see a small whiff of condensation on the stove. This is the water in the gas and at a burner tip, can not cause any problems. But compress this same gas and use it for fueling your natural gas powered vehicle and it is a different story.

Gas companies typically deliver gas with a 7#MMSCF which means 7 pounds of water per million cubic feet of gas. This is sufficiently low enough to avoid condensation, hydrate formation and freeze-ups at normal pipeline operating pressures. But 7#/MMSCF water content will cause problems for your refueling station and vehicle.

Many times we asked just how dry does the natural gas have to be. Gas companies typically deliver gas with a 7#MMSCF which means 7 pounds of water per million cubic feet of gas. This is sufficiently low enough to avoid condensation, hydrate formation and freeze-ups at normal pipeline operating pressures.

But, what does 7#/MMSCF mean for NGV applications? It means a +52 degree F. Dew point @ 3,600 psi. This can translate into potential condensation, hydrate formation and freeze ups during gas expansion due to the Joule – Thomson effect.

To eliminate any potential problems for your station or your customers, the gas should be dried to a pressure dew point (PDP) that is well below the minimum ambient temperature that will occur at the highest storage pressure.

The society of Automotive Engineers has issued a fuel specification (SAE J1616) that specifies the gas be dried to a local dew point temperature of -10 degrees F. below the 99.0% winter design dry-bulb temperature as found in the 1989 ASHRAE handbook at the maximum operating tank pressure.

In northern parts of US the winter design dry bulb temperature is 1 degree F., therefore, operators must dry the gas to a PDP of -11 degree F in order to meet fuel spec J1616. In a South of US the winter design dry bulb temperature is 35 degrees F., we then need to dry to a PDP of 25 degree F to meet J1616.